This patent application is directed at a current collector that works with an anode electrode (anode active material layer) or a cathode electrode (cathode active material layer) of a lithium cell (e.g. lithium-ion cell, lithium-metal cell, or lithium-ion capacitor), a supercapacitor, a non-lithium battery (such as the zinc-air cell, nickel metal hydride battery, sodium-ion cell, and magnesium-ion cell), and other electrochemical energy storage cells. This application is not part of the anode active material layer or the cathode active material layer per se.
The lithium-metal cell includes the conventional lithium-metal rechargeable cell (e.g. using a lithium foil as the anode and MnO2 particles as the cathode active material), lithium-air cell (Li-Air), lithium-sulfur cell (Li—S), and the emerging lithium-graphene cell (Li-graphene, using graphene sheets as a cathode active material), lithium-carbon nanotube cell (Li—CNT, using CNTs as a cathode), and lithium-nano carbon cell (Li—C, using nano carbon fibers or other nano carbon materials as a cathode). The anode and/or the cathode active material layer can contain some lithium, or can be prelithiated prior to or immediately after cell assembly.
Rechargeable lithium-ion (Li-ion), lithium metal, lithium-sulfur, and Li metal-air batteries are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest lithium storage capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound as an anode active material (except Li4.4Si, which has a specific capacity of 4,200 mAh/g). Hence, in general, Li metal batteries (having a lithium metal anode) have a significantly higher energy density than conventional lithium-ion batteries (having a graphite anode).
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having relatively high specific capacities, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte and the cathode became lithiated. Unfortunately, upon repeated charges and discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately caused internal shorting, thermal runaway, and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries. Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries (e.g. Lithium-sulfur and Lithium-transition metal oxide cells) for EV, HEV, and microelectronic device applications.
Prompted by the aforementioned concerns over the safety of earlier lithium metal secondary batteries led to the development of lithium-ion secondary batteries, in which pure lithium metal sheet or film was replaced by carbonaceous materials (e.g. natural graphite particles) as the anode active material. The carbonaceous material absorbs lithium (through intercalation of lithium ions or atoms between graphene planes, for instance) and desorbs lithium ions during the re-charge and discharge phases, respectively, of the lithium-ion battery operation. The carbonaceous material may comprise primarily graphite that can be intercalated with lithium and the resulting graphite intercalation compound may be expressed as LixC6, where x is typically less than 1 (with graphite specific capacity <372 mAh/g).
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost, safety, and performance targets (such as high specific energy, high energy density, good cycle stability, and long cycle life). Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of lithium transition-metal oxide or phosphate based cathode active material is typically in the range of 140-170 mAh/g. As a result, the specific energy (gravimetric energy density) of commercially available Li-ion cells featuring a graphite anode and a lithium transition-metal oxide or phosphate based cathode is typically in the range of 120-220 Wh/kg, most typically 150-200 Wh/kg. The corresponding typical range of energy density (volumetric energy density) is from 400 to 550 Wh/L. The energy densities are even lower under high charge-discharge rate conditions. These specific energy values are two to three times lower than what would be required if battery-powered electric vehicles are to be widely accepted.
A typical battery cell is composed of (a) an anode current collector, (b) an anode electrode (also referred to as the anode active material layer, typically including an anode active material, a conductive filler, and a binder resin component) bonded to the anode current collector with a binder resin, (c) an electrolyte/separator, (d) a cathode electrode (also referred to as the cathode active material layer, typically including a cathode active material, a conductive filler, and a binder resin), (e) a cathode current collector bonded to the cathode electrode with a binder resin, (f) metal tabs that are connected to external wiring, and (g) casing that wraps around all other components except for the tabs.
Current collectors, typically aluminum foil (at the cathode) and copper foil (at the anode), account for about 15-20% by weight and 10-15% by cost of a lithium-ion battery. Therefore, thinner, lighter foils would be preferred. However, there are several major issues associated with state-of-the-art current collectors: (a) Due to easy creasing and tearing, thinner foils tend to be more expensive and harder to work with; (b) Current collectors must be electrochemically stable with respect to the cell components over the operating potential window of the electrode. In practice, continued corrosion of the current collectors mainly by the electrolyte can lead to a gradual increase in the internal resistance of the battery, resulting in persistent loss of the apparent capacity or poor cycling life: (c) Oxidation of metal current collectors is a strong exothermic reaction that can significantly contribute to thermal runaway of a lithium battery.
Accordingly, the current collectors are crucially important for cost, weight, safety, and performance of a battery. Instead of metals, graphene or graphene-coated solid metal or plastic has been considered as a potential current collector material, as summarized in the references listed below:    1. Li Wang, Xiangming He, Jianjun Li, Jian Gao, Mou Fang, Guangyu Tian, Jianlong Wang, Shoushan Fan, “Graphene-coated plastic film as current collector for lithium/sulfur batteries,” J. Power Source, 239 (2013) 623-627.    2. S. J. Richard Prabakar, Yun-Hwa Hwang, Eun Gyoung Bae, Dong Kyu Lee, Myoungho Pyo, “Graphene oxide as a corrosion inhibitor for the aluminum current collector in lithium ion batteries,” Carbon, 52 (2013) 128-136.    3. Yang Li, et al. Chinese Patent Pub. No. CN 104600320 A (2015 May 6).    4. Zhaoping Liu, et al (Ningbo Institute of Materials and Energy, China), WO 2012/151880 A1 (Nov. 15, 2012).    5. Gwon, H.; Kim, H-S; Lee, K E; Seo, D-H; Park, Y C; Lee, Y-S; Ahn, B T; Kang, K “Flexible energy storage devices based on graphene paper,” Energy and Environmental Science. 4 (2011) 1277-1283.    6. Ramesh C. Bhardwaj and Richard M. Mank, “Graphene current collectors in batteries for portable electronic devices,” US 20130095389 A1, Apr. 18, 2013.
Currently, graphene current collectors come in three different forms: graphene-coated substrate [Ref. 1-4], free-standing graphene paper [Ref. 5], and monolayer graphene film produced by transition metal (Ni, Cu)-catalyzed chemical vapor deposition (CVD) followed by metal etching [Ref. 6].
In the preparation of graphene-coated substrate, small isolated sheets or platelets of graphene oxide (GO) or reduced graphene oxide (RGO) are spray-deposited onto a solid substrate (e.g. plastic film or Al foil). In the graphene layer, the building blocks are separated graphene sheets/platelets (typically 0.5-5 μm in length/width and 0.34-30 nm in thickness) that are typically bonded by a binder resin, such as PVDF [Refs. 1, 3, and 4]. Although individual graphene sheets/platelets can have a relatively high electrical conductivity (within the confine of that 0.5-5 μm), the resulting graphene-binder resin composite layer is relatively poor in electrical conductivity (typically <100 S/cm and more typically <10 S/cm). Furthermore, another purpose of using a binder resin is to bond the graphene-binder composite layer to the substrate (e.g. Cu foil); this implies that there is a binder resin (adhesive) layer between Cu foil and the graphene-binder composite layer. Unfortunately, this binder resin layer is electrically insulating and the resulting detrimental effect seems to have been totally overlooked by prior workers.
Although Prabakar, et al. [Ref 2] does not seem to have used a binder resin in forming an aluminum foil coated with discrete graphene oxide sheets, this graphene oxide-coated Al foil has its own problem. It is well-known in the art that aluminum oxide (Al2O3) readily forms on surfaces of an aluminum foil and cleaning with acetone or alcohol is not capable of removing this passivating layer of aluminum oxide or alumina. This aluminum oxide layer is not only electrically and thermally insulating, but actually is not resistant to certain types of electrolyte. For instance, the most commonly used lithium-ion battery electrolyte is LiPF6 dissolved in an organic solvent. A trace amount of H2O in this electrolyte can trigger a series of chemical reactions that involve formation of HF (a highly corrosive acid) that readily breaks up the aluminum oxide layer and continues to corrode the Al foil and consume electrolyte. The capacity decay typically becomes much apparent after 200-300 charge-discharge cycles.
Free-standing graphene paper is typically prepared by vacuum-assisted filtration of GO or RGO sheets/platelets suspended in water. In a free-standing paper, the building blocks are separated graphene sheets/platelets that are loosely overlapped together. Again, although individual graphene sheets/platelets can have a relatively high electrical conductivity (within the confine of that 0.5-5 μm), the resulting graphene paper has a very low electrical conductivity; e.g. 8,000 S/m or 80 S/cm [Ref. 5], which is 4 orders of magnitude lower than the conductivity of Cu foil (8×105 S/cm).
There are several major problems associated with the most commonly used process for producing graphene (i.e. the chemical oxidation/intercalation process):                (1) The process requires the use of large quantities of several undesirable chemicals, such as sulfuric acid, nitric acid, and potassium permanganate or/and sodium chlorate.        (2) The thermal exfoliation requires a high temperature (typically 800-1,050° C.) and, hence, is a highly energy-intensive process.        (3) The approach requires a very tedious washing and purification step. For instance, typically 2.5 kg of water is used to wash and recover 1 gram of GIC, producing huge quantities of waste water that need to be properly treated.        (4) The resulting products are graphene oxide (GO) platelets that must undergo a further chemical reduction treatment to reduce the oxygen content. Typically even after reduction, the electrical conductivity of GO platelets remains much lower than that of pristine graphene. Furthermore, the reduction procedure often involves the utilization of toxic chemicals, such as hydrazine.        (5) Furthermore, the quantity of intercalation solution retained on the flakes after draining may range from 20 to 150 parts of solution by weight per 100 parts by weight of graphite flakes (pph) and more typically about 50 to 120 pph. During the high-temperature exfoliation, the residual intercalate species retained by the flakes decompose to produce various species of sulfuric and nitrous compounds (e.g., NOx and SOx), which are undesirable. The effluents require expensive remediation procedures in order not to have an adverse environmental impact.        
The catalyzed CVD process for graphene production involves introduction of a hydrocarbon gas into a vacuum chamber at a temperature of 500-800° C. Under these stringent conditions, the hydrocarbon gas gets decomposed with the decomposition reaction being catalyzed by the transition metal substrate (Ni or Cu). The Cu/Ni substrate is then chemically etched away using a strong acid, which is not an environmentally benign procedure. The whole process is slow, tedious, and energy-intensive, and the resulting graphene is typically a single layer graphene or few-layer graphene (up to 5 layers maximum since the underlying Cu/Ni layer loses its effectiveness as a catalyst).
Bhardwaj, et al [Ref. 6] suggested stacking multiple CVD-graphene films to a thickness of 1 μm or a few μm; however, this would require hundreds or thousands of films stacked together (each film being typically 0.34 nm to 2 nm thick). Although Bhardwaj, et al claimed that “The graphene may reduce the manufacturing cost and/or increase the energy density of a battery cell,” no experimental data was presented to support their claim. Contrary to this claim, the CVD graphene is a notoriously expensive process and even a single-layer of CVD graphene film would be significantly more expensive than a sheet of Cu or Al foil given the same area (e.g. the same 5 cm×5 cm). A stack of hundreds or thousands of mono-layer or few-layer graphene films as suggested by Bhardwaj, et al would mean hundreds or thousands times more expensive than a Cu foil current collector. This cost would be prohibitively high. Further, the high contact resistance between hundreds of CVD graphene films in a stack and the relatively low conductivity of CVD graphene would lead to an overall high internal resistance, nullifying any potential benefit of using thinner films (1 μm of graphene stack vs. 10 μm of Cu foil) to reduce the overall cell weight and volume. It seems that the patent application of Bhardwaj, et al [Ref. 6], containing no data whatsoever, is nothing but a concept paper.
The above discussions have clearly shown that all three forms of the graphene-enhanced or graphene-based current collector do not meet the performance and cost requirements for use in a battery or supercapacitor. A strong need exists for a different type of material for use as a current collector.
The present invention is directed at a new class of materials, herein referred to as a highly oriented film of humic acid (HA), alone or in combination with graphene, which is chemically bonded to metal foil surface. Graphene used herein includes pristine graphene, graphene oxide, graphene fluoride, nitrogenated graphene, hydrogenated graphene, boron-doped graphene, any other type of doped graphene, and other type of chemically functionalized graphene. Quite unexpectedly and significantly, this highly oriented film of HA or HA/graphene mixture can be thermally converted to a highly conducting graphitic film.
Humic acid (HA) is an organic matter commonly found in soil and can be extracted from the soil using a base (e.g. KOH). HA can also be extracted, with a high yield, from a type of coal called leonardite, which is a highly oxidized version of lignite coal. HA extracted from leonardite contains a number of oxygenated groups (e.g. carboxyl groups) located around the edges of the graphene-like molecular center (SP2 core of hexagonal carbon structure). This material is slightly similar to graphene oxide (GO) which is produced by strong acid oxidation of natural graphite. HA has a typical oxygen content of 5% to 42% by weight (other major elements being carbon and hydrogen). HA, after chemical or thermal reduction, has an oxygen content of 0.01% to 5% by weight. For claim definition purposes in the instant application, humic acid (HA) refers to the entire oxygen content range, from 0.01% to 42% by weight. The reduced humic acid (RHA) is a special type of HA that has an oxygen content of 0.01% to 5% by weight.
It is surprising to discover that humic acid, when brought in intimate contact with a surface of a metal foil, can chemically bond to the metal foil. It is further surprising to discover that, when properly aligned and packed together, humic acid molecules can chemically link with one another to obtain longer and wider humic acid sheets. These humic acid molecules are also capable of chemically linking or bonding with graphene sheets, if present and properly aligned and packed. The resulting humic acid- or graphitic film-bonded thin metal foil is electrolyte-compatible, non-reactive, corrosion-protective, of low contact resistance, thermally and electrically conductive, ultra-thin, and light-weight, enabling a battery or capacitor to deliver a higher output voltage, higher energy density, high rate-capability, and much longer cycle life.